Haloa japonica (Pilsbry, 1895) is a cephalaspidean bubble snail of the family Haminoeidae Pilsbry, 1895, native to the northwestern Pacific. Haloa japonica has been introduced to the Mediterranean Sea and western North America, where it can be locally abundant in estuarine habitats. To predict the future spread and establishment of this non-native species, it is important to understand how the species responds to and is tolerant of fluctuating salinity levels, one of the most important limiting factors for this species. In this study, we tested the salinities at which specimens of Haloa japonica were able to survive and maintain normal life functions, either by direct or gradual transfer of specimens into different salinities. The results showed that specimens were able to survive at least three days after direct transfer from 35 ppt into 20 ppt higher or lower salinities. Gradual transfer of specimens into different salinities showed that all specimens survive and behave normally in water with salinity in the range of 20–45 ppt, although survival decreases at extreme salinities. In cases of extreme salinities, we also observed lower activity of Haloa japonica and a change in body shape. Furthermore, higher salinities were more lethal than lower salinities. Based on the feeding rate and faecal production of Haloa japonica, the optimum salinity ranges from 30–40 ppt. The results showed that Haloa japonica is well adapted to salinity fluctuations, thus enabling the species to survive in extreme environments, to disperse by human-mediated transport, and to colonise new areas.